In audio (not transportation) terms, a Bus is a point in a circuit where many signals are brought together. For example: Most electronic items have a Ground Bus where all of a device’s individual ground paths are tied together. In mixers, we have Mix Busses, where multiple channels’ signals are brought (or blended) together; Aux Busses, where feeds from channels are brought together to be routed to an external processor or monitor send, etc. In general, the more busses a mixer has, the more flexible the routing capabilities of that mixer will be.